The non-contact measurement of electromagnetic properties of a metal material such as magnetic permeability, core loss and electric conductivity or a quantity of the metal material which has correlation to electromagnetic properties is used for various purposes. For example, in paragraph [0015] of Japanese Patent 2519615, there is the description on an example where iron loss is measured by a known method in such a manner that a primary coil and a secondary coil for measuring core loss are arranged in a manufacturing line of a grain-oriented electrical steel sheet (between an annealing furnace and an annealing separator coating device or during a period in which the annealing separator is applied by coating, is dried and is wound into a coil shape), and a steel sheet is made to pass through these coils. It is thought that, in this method, large-sized coils are used and an average core loss in the widthwise direction of the steel sheet is measured using an alternate current magnetic flux.
Further, in JP-A-53-20986, there is the description where an alternate current magnetic flux is applied to an object to be measured (an electric conductive object such as iron slab or hot rolled strip), and a change in electric conductivity and a change in magnetic permeability depending on a temperature of an object to be measured are measured by measuring a magnetic field which is generated by an interaction between the magnetic flux and the object to be measured, and the temperature of the object to be measured is measured eventually.
As sensors which serve for such measurement, sensors of various configurations are conceivable. Among these sensors, a sensor having a U-shaped core is one kind of general use sensor. For example, JP-A-8-36038 discloses an example of such a sensor for measuring magnetic permeability.
A method which evaluates magnetic property or the like by detecting an orientation of crystal grains, for example, using ultrasonic waves instead of detecting magnetic property per se is also considered as a non-contact means (although water is interposed).
However, the above-mentioned prior art has a drawback that local magnetic property within a range from several mm to several 10 mm cannot be measured with high precision while minimally being influenced by disturbances or the like.
The usual magnetic property measurement is generally performed in a domain wall motion region (or domain wall displacement region) where the difference in property between a sound portion which has achieved desired magnetic property and an unsound portion which has not yet achieved the desired magnetic property is extremely large. In the domain wall motion region, the magnetic property is also strongly influenced by factors such as a particle diameter, precipitates, stress (tension) which possibly become error factors. Further, the magnetic property is largely influenced by a plate edge (the plate edge being a portion where the property of a ferromagnetic material and the property of a non-magnetic material (air) are discontinuous thus forming a dead zone at an edge) or a change in liftoff (distance between the sensor and an object to be measured).
This is because, in the domain wall motion region, the differential magnetic permeability is large and a change in the differential magnetic permeability attributed to the fluctuation of measuring conditions is also large. Hence, a sensor output is largely changed due to the presence or the non-presence of the object to be measured (influence exerted by the plate edge) and a distance between the sensor and the object to be measured (influence of the change in liftoff). Under such circumstances, it has been difficult to realize the measurement with high precision, particularly, the stable measurement in on-line (in a manufacture line).
The method which uses ultrasonic waves may be influenced by a trivial change in shape of a steel sheet. Hence, the improvement of the precision of measurement is also desired.
It could therefore be helpful to provide a magnetism measuring method and device which can measure the local magnetic property of a magnetic material with high precision while being minimally influenced by disturbances or the like.